A number of closed-cycle thermodynamic machines operate by applying heat from a high-grade heat source to a heat user, such as a steam boiler or an external combustion thermodynamic heat engine. One example of such a heat user is a Stirling engine. Typically, this is accomplished by using a heat source in the form of a fossil fuel burner, or a concentrated solar energy source, to directly heat a heat exchanger integral with the heat user.
However, combustion systems that directly impinge on the heater heads of Stirling engines typically deliver heat to them at non-uniform temperatures. As a result, the non-uniform temperatures can compromise interior working gas bulk temperatures, causing the engine performance to drop. Additionally, local hot spots can excessively heat the engine structural materials.
One type of heater head that directly delivers heat to the heater head of a Stirling engine is a fossil fuel burner. One area where fossil fuel burners have been developed is where fossil fuel burners are used to deliver a primary heat source to a Stirling engine. Another area is as secondary, or backup heat sources where fossil fuel augments solar energy of a solar-powered electric system. Irrespective of whether they are supplied with burners used as primary or secondary heat sources, Stirling engines require the delivery of concentrated thermal energy at uniform temperature to the engine working fluid. Uniform temperatures enable a Stirling engine to realize maximum working efficiency, while past systems have mainly utilized engine burners that transfer most or all of the heat by convection. Furthermore, a typical Stirling engine has a heater head composed of many parallel, small-diameter tubes. The burner, or burner assembly, must accommodate such a heater head shape, usually resulting in a non-conforming interface that produces a poor interface between the burner and the engine head. As a result, many past systems utilizing direct impingement burners have resulted in poor performance that suffers from inherent non-uniformity of temperature. Particularly where the Stirling engine has a heater head composed of many parallel, small-diameter tubes, hot spot burnout is a substantial operating risk, particularly taking into account the long-term operation of the exposed heater tubes and brazed joints of heater heads within an oxidizing atmosphere.
In the approach presented in this disclosure, an improved burner assembly provides a burner assembly to heater head interface that transfers heat primarily by radiation, and secondarily by convection. The heater head utilizes commercially available low-emission burner technology. The externally positioned burner assembly delivers heat energy directly to the heater head along an improved interface that significantly reduces the risk of hot spots, greatly contributing to the extended life of the heater head. In this manner, operating performance can be maximized for a given peak metal temperature. Furthermore, exhaust gas energy can be effectively recovered to achieve high overall operating efficiency.